This invention relates to an improved vibratory viscometer transducer and circuit for measuring the viscosity of fluids, which is particularly suitable for, but not limited to in-line process control applications; and to an in-line instrument capable of measuring both the viscosity and the density of a fluid.
Rotational vibratory viscometers are well known in the art, and generally comprise (i) a transducer having a tip immersible in a fluid the viscosity of which is to be determined, (ii) an electromagnetic drive coil for causing the tip to rotationally oscillate with a very small angular amplitude, (iii) a feedback control circuit for maintaining the angular amplitude of oscillation of the tip at a predetermined constant value irrespective of the viscosity of the fluid, and (iv) a circuit for determining the power supplied to the drive coil, usually by squaring the current supplied to said coil, which power is a measure of the viscosity of the fluid.
A viscometer of this type is described, for example, in "Viscometer for Energy Saving", J. V. Fitzgerald, F. J. Matusik, and P. C. Scarna, Jr., Measurements & Control April 1980. Similar viscometers are described in the references cited in said article, as well as in U.S. Pat. Nos. 3,382,706; 3,710,614; 3,712,117; 3,672,429; 3,875,791; and 4,299,119; and in copending U.S. patent application Ser. No. 483,142, filed Apr. 8, 1983 and assigned to the assignee of the present application.
Such viscometers, however, are not well suited for in-line process control applications where the viscosity of a fluid flowing in a pipe has to be continuously monitored, in that the prior art vibratory viscometers require immersion of the transducer tip in the fluid stream, resulting in undesirable turbulence and restriction of flow. In cases where the fluid stream contains suspended particles, these tend to build up on the viscometer tip, altering its characteristics. Further, since vibratory viscometers inherently measure the viscosity-density product of the fluid, in order to determine actual viscosity it is necessary to determine the density of the fluid; and prior art vibratory viscometers require either a manually set density input or a signal from a separate densitometer.
Accordingly, an object of the present invention is to provide an improved vibratory viscometer transducer and circuit suitable for in-line process control applications.